On Oct 30, 2008, at 6:36 AM, John Baldwin wrote:

Ok, so it's not a PMBR. My understanding is that a GPT requires the MBR to be a PMBR (only one partition in the 4th slot with a special type of 0xee that covers the whole disk). What this box is doing is trying to make the MBR match the first 4 partitions in the GPT. I'm not sure if you will be able to get FreeBSD's GPT stuff to recognize that reliably. Marcel (cc'd) might have
some ideas.

In FreeBSD 6, the kernel checks explicitly for a PMBR when it checks
for a GPT. Besides being part of the specification, it also avoids
conflicts. In the GEOM framework, there's no a priori support for
having one GEOM control another. When there's a valid MBR as well as
a valid GPT, it's undeterministic which one will be used, unless
they both cooperate. They don't.

This is where GPart helps out. In FreeBSD 7 and up, GPart supports
multiple partitioning schemes, including MBR and GPT. The kernel
will not enforce a PMBR in front of a GPT, because upon detecting
both a MBR and a GPT, the GPT will be used. However, this only
applies when the kernel is configured with GEOM_PART_MBR and not
with GEOM_MBR. At this time GEOM_MBR is still the default.

So, to make it work for you, you need at least FreeBSD 7.1 (to
be released shortly) or use next month's snapshot and build a
custom kernel without GEOM_MBR and with GEOM_PART_MBR.

In FreeBSD 8 and up GPart is the default and you won't have to make
a custom kernel in that case.


Marcel Moolenaar

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